Present day sliplasted and boardlasted running and jogging shoes, as well as other types of athletic shoes, are customarily manufactured with a laminate bottom construction having a closed cell polymeric foam midsole overlying and adhered to a flexible outsole. Sliplasted athletic running shoes mainly differ from boardlasted athletic shoes in that they have no insole board and instead have a closed fabric bottom to extend completely around the foot like a slipper. In the boardlasted athletic running shoes, the upper has an open bottom which is closed by an insole board overlying the midsole. The foamed midsole is used in both types of shoes because of its lightness and shock absorbing properties to cushion the wearer's foot against impacts during running.
Sliplasted athletic shoes are generally preferred over boardlasted athletic shoes because they usually are more comfortable than boardlasted athletic shoes. Due to the absence of an insole board, however, sliplasted athletic shoes are usually less stable than boardlasted athletic shoes because concentrated stresses are developed in the foamed midsole during running to non-uniformly degrade the foamed midsole to an objectionable extent. The foamed midsole therefore loses much of its effectiveness as a shock absorber because of the non-uniform degradation.
Degradation of the foamed midsole arises from continual stresses which are developed by the repeated and cyclic deformation and relaxation of the midsole during the course of running. The degradation may take the form of a permanent compressive deformation or compression set, as it is called, of the midsole. Degradation may also occur without compression set where there is a breakdown or loss of the closed cells in the foamed midsole. One cause of such a cellular breakdown is the lateral outward expansion of the midsole due to compressive forces acting on the midsole.
Midsole degradation is frequently localized due to concentrated force patterns in the midsole's impact absorbing regions, particularly in the heel or rearfoot region under the wearer's calcaneus. Concentrated force patterns may also be set up in other regions due to a runner's particular running style. For example, localized midsole degradation may occur at the rear outside border of the shoe in the region of the back of the heel.
Localized midsole degradation along either the medial inside or lateral outside shoe edges under the heel is particularly troublesome because it tends to cause the shoe to lose its stability by tilting as the runner's foot strikes the ground. To compensate for the instability, the runner may adjust his running style which, in turn, could lead to foot or ankle injuries. Furthermore, the problem of midsole degradation is compounded by the fact that it frequently occurs before the outsole loses its service life.
The foamed midsole in a boardlasted athletic running shoe will also degrade as a result of usage, but the degradation is not as nonuniform as the midsole degradation in a slip-lasted athletic shoe because the insole board of the boardlasted athletic shoe more uniformly distributes the forces acting on the midsole to some extent as long as the insole board itself does not degrade.
Although an insole board does distribute the forces acting on the foamed midsole to some extent, it was found that if the insole board is made stiff enough to adequately spread the forces over the foamed midsole, the shoe becomes too hard, causing appreciable discomfort to the wearer. On the other hand, a low strength insole board which meets the wearer's comfort requirements is likely to degrade, thus giving rise to excessive nonuniform degradation even in boardlasted shoes.